Membranes new

Biological membranes and Biological membranes and bioelectric phenomenabioelectric phenomena

A part of this lecture was prepared on the basis of a presentation kindly provided by Prof. Katarína Kozlíková from the Dept. of Medical Biophysics, Medical Faculty, Comenius University in Bratislava

A part of this lecture was prepared on the basis of a presentation kindly provided by Prof. Katarína Kozlíková from the Dept. of Medical Biophysics, Medical Faculty, Comenius University in Bratislava

Lectures on Medical Biophysics

Dept. Biophysics, Medical faculty, Masaryk University in Brno

Biological membrane

• It is not possible to understand the origin of resting and action membrane voltage (potential) without knowledge of structure and properties of biological membrane.

• In principle, it is an electrically non-conducting thin bilayer (6-8 nm) of phospholipid molecules. There are also built-in macromolecules of proteins with various functions. Considering electrical phenomena, two kinds of proteins are the most important: the ion channels and pumps. In both cases these are components of transport mechanisms allowing transport of ions through the non-conducting phospholipid membrane.

Bioelectric phenomena

• The electric signal play a key role in controlling of all The electric signal play a key role in controlling of all vitally important organs. They ensure fast vitally important organs. They ensure fast transmission of information in the organism. They transmission of information in the organism. They propagate through nerve fibres and muscle cells propagate through nerve fibres and muscle cells where they trigger a chain of events resulting in where they trigger a chain of events resulting in muscle contraction. They take a part in basic function muscle contraction. They take a part in basic function mechanisms of sensory and other body organs. mechanisms of sensory and other body organs.

• On cellular level, they originate in membrane systems, On cellular level, they originate in membrane systems, and their propagation is accompanied by production and their propagation is accompanied by production of electromagnetic field in the ambient medium. of electromagnetic field in the ambient medium.

• Recording of electrical or magnetic signals from the Recording of electrical or magnetic signals from the body surface is fundamental in many important body surface is fundamental in many important clinical diagnostic methods.clinical diagnostic methods.

Structure of the membrane

Phospholipid bilayer

Phospholipid bilayer

Integral proteinsIntegral proteins

Channels• The basic mechanism of the ion exchange between

internal and external medium of the cell are the membrane channels. They are protein molecules but, contrary to the pumps with stable binding sites for the transmitted ions, they form water-permeable pores in the membrane. Opening and closing of the channels (gating) is performed in several ways. Besides the electrical gating we can encounter gating controlled by other stimuli in some channels (chemical binding of substances, mechanical tension etc.).

• The passage of ions through the whole channel cannot be considered to be free diffusion because most channels are characterised by certain selectivity in ion permeability. Sodium, potassium, calcium or chloride channels are distinguished.

• In this kind of ion transport there is no need of energy delivery.

Electrical and chemical gating

polarised membrane depolarisedclosed channel open channelpolarised membrane depolarisedclosed channel open channel

closed openchannel channelclosed open

channel channel

Ion transport systems• Many ion transport systems were discovered in cell

membranes. One of them, denoted as sodium-potassium pump (Na/K pump) has an extraordinary importance for production of membrane voltage. It removes Na-ions from the cell and interchanges them with K-ions. Thus, the concentrations of these ions in the intracellular and extracellular medium (they are denoted as [Na+], [K+] and distinguished by indexes i, e) are different. We can write:

.][][,][][ eiie KKNaNa Working Na/K pump requires constant energy consumption. This energy is delivered to the transport molecules by the adenosine triphosphate (ATP) which is present in the intracellular medium.

Principle of the sodium-potassium pump

The sodium ions are released on the outer side of the membrane. Following conformation change of the ion pump molecule enables binding of potassium ions which are carried inside the cell.

Function of biological membranes

• They form the interface between the cells and also between cell compartments.

• They keep constant chemical composition inside bounded areas by selective transport mechanisms.

• They are medium for fast biochemical turnover done by enzyme systems.

• Their specific structure and selective ion permeability is a basis of bioelectric phenomena.

Excitability Characteristic feature of living systems on any level of organisation of living matter

An important condition of adaptation of living organisms to environment

An extraordinary ability of some specialised cells (or tissues – muscle cells, nerve cells)

Each kind of excitable tissue responses most easily on a certain energetic impulse (the adequate stimulus). Another energetic impulse can also evoke an excitation but much more energy is necessary (the inadequate stimulus).

Resting membrane potential

Resting membrane potential

membrane

Resting membrane potential – RMP (1)

Potential difference between a microelectrode inside the cell (negative potential) and a surface electrode outside the

cell (zero potential) = membrane voltage = membrane = membrane voltage = membrane potentialpotential

„Non-polarisable“ electrodes are used

membrane

Extracellular space

Extracellular space

Intracellular space

Its values depend on:- Type of the cell - Art of the animal the cell is taken from- For identical cells – on the composition and concentration

of the ion components of the extracellular liquids

The value of RMP at normal ion composition of the IC and EC liquid: (-100 mV to -50 mV)

Resting membrane potential – RMP (2)

Membrane thickness ~ 10 nm

Electric field intensity in the membrane ~ 107 V/m

Electric field intensity on the Earth’s surface ~ 102 V/m

Approach to the RMP (1): Electrodiffusion models: They describe processes

phenomenologically on the basis of thermodynamics. Origin of the RMP is connected with diffusion of ions across the membrane - Nernst and Donnan models, ion transport model

(2): Physical – based on description of behaviour of solids or liquid crystals

- describe movement of ions across the membrane and its blocking

- they consider characteristic properties of structural elements of the membrane (lipids, proteins)

(3) Models based on equivalent electrical circuits: They describe behaviour of the cells in rest or excited state. Electrical properties of the cells are considered in accord with other models.

Diffusion potential DP (1)Caused by diffusion of charged particles DP in non-living systems – solutions are separated by a membrane permeable for Na+ and Cl-.

The compartments are electroneutral, but there is a concentration gradient

Diffusion of ions from [1] do [2]

Hydration envelope (water molecules are bound to ions) Na+ (more) a Cl- (less) faster diffusion of Cl- against (!) concentration gradient Transient voltage appears across the two compartments Diffusion potential

Electric field repulses Cl- from [2]

Diffusion potential DP (2)DP in living systems – the solutions are separated by a selectively permeable membrane for K+ (right), non-permeable for pro Na+ a Cl-.

In such a system, an equilibrium arises if there is no resulting flux of ions.

Diffusion of K+ against its concentration gradient occurs until an electric gradient of the same magnitude, but of opposite direction arises An equilibrium potential emerges – resulting diffusion flux is equal to zero

membrane

Electrolyte I

anions cAIIcations cC

I

A simple example of a membrane equilibrium (1)

Electrolyte II

cations cCII anions cA

I

The same electrolyte is on both sides of the membrane but of different concentrations (cI > cII), the membrane is permeable only for cations

Result:Electric double layer is

formed on the membranelayer 1: anions stopped in

space Ilayer 2: cations attracted to

the anions (II)

membrane

Electrolyte I


I

A simple example of a membrane equilibrium (2)

Electrolyte II


I

The concentration difference ”drives” the cations, electric field of the bilayer “pushes them back”

In equilibrium: potential difference U arises:

IIC

IC

C

III

c

c

Fz

TR

U

ln

I II

---------

+++++++++

(Nernst equation)

membrane

Electrolyte I

anions R-


I

Electrolyte II


I

Donnan equilibrium (1)The same electrolyte is on both sides, concentrations are different (cI > cII), membrane is permeable for small univalent ions C+ and A-, non-permeable for R- .

Diffusible ions: C+, A- diffuse freely non-diffusible ions: R-

In presence of R-: Equal distribution of C+ and A- cannot be achieved a special case of equilibrium - Donnan equilibrium

membrane

Electrolyte I

anions R-


I

Electrolyte II


I

Donnan equilibrium (2)

Equilibrium concentrations: IIA

IIC

IA

IC cccc

Donnan ratio:

rc

c

c

cIA

IIA

IIC

IC

membrane

Electrolyte I

anions R-


I

Electrolyte II


I

Donnan equilibrium (3)

-----------

+++++++++++

Donnan ratio:

rc

c

c

cIA

IIA

IIC

IC

rF

TR

c

c

F

TR

c

c

F

TR

U

IA

IIA

IIC

IC

III

ln

ln

ln

Donnan potential:

izIIi

Ii

c

cr

cell membrane

intra extra

phosphateanions

proteinanions

Na+

Cl-

K+

K+

Cl-

Donnan model in living cell (1)

diffuse: K+, Cl- do not diffuse: Na+, anions,

also proteins and nucleic

acids

Concentrations:

[K+] in > [K+] ex

[Cl-] in < [Cl-] ex

Cell membrane

intra extra

phosphateanions

proteinanions

Na+

Cl-

K+

K+

Cl-

in

ex

ex

in

Cl

Cl

K

K

Donnan ratio:

Donnan potential:

in

ex

ex

in

exin

Cl

Cl

F

TR

K

K

F

TR

U

ln

ln

-----------

+++++++++++


Donnan potential (resting potential) [mV]:object: calculated: measured:

K+: Cl-:cuttlefish axon - 91 - 103 - 62frog muscle - 56 - 59 - 92rat muscle - 95 - 86 - 92


Donnan model differs from reality:The cell and its surroundings are regarded as closed thermodynamic systemsThe diffusible ions are regarded as fully diffusible, the membrane is no barrier for the diffusible ionsThe effect of ionic pumps on the concentration of ions is neglected The interaction between membrane and ions is not considered

Model of ion transport (1)

We suppose:A constant concentration difference between outer and inner side of the membrane constant transport rate through membraneMigration of ions through membrane electric bilayer on both sides of the membraneAll kinds of ions on the both sides of the membrane are considered simultaneouslyEmpirical fact – membrane is neither fully permeable nor fully non-permeable for any ionDifferent ions have different permeability

Electrodiffusion model with smaller number of simplifications.

Model of ion transport (2)GoldmanGoldman - Hodgkin - Katz - Hodgkin - Katz

P - permeability

k = cations, a = anionsk = cations, a = anions

Model of ion transport (3)

„giant“ cuttlefish axon (t = 25°C): ppKK : p : pNaNa : p : pClCl = 1 : 0.04 : 0.45 = 1 : 0.04 : 0.45

calculated: U = - 61 mV measured: U = - 62 mV

frog muscle (t = 25°C): ppKK : p : pNaNa : p : pClCl = 1 : 0.01 : 2 = 1 : 0.01 : 2

calculated: U = - 90 mV measured: U = - 92 mV

Action

potential

Action potential

• The concept of action potential denotes a fast change of the resting membrane potential caused by over-threshold stimulus which propagates into the adjacent areas of the membrane.

• This potential change is connected with abrupt changes in sodium and potassium ion channels permeability.

• The action potential can be evoked by electrical or chemical stimuli which cause local decrease of the resting membrane potential.

Mechanism of action potential triggering

tUm

UNa

Upr

UK

Umr

0

t

APDepolarization phasePositive feedback: gNa depol gNa

Repolarization phase:inactivation gNa and

activation gK

hyperpolarization (deactivation gK)

Mechanism of the action potential triggering in the cell membrane is an analogy of a monostable flip-flop electronic circuit.

Origin of action potential Origin of action potential

Action potential

• Changes in the distribution of ions caused by action potential are balanced with activity of ion pumps (active transport).

• The action potential belongs among phenomena denoted as „all or nothing“ response. Such response is always of the same size. Increasing intensity of the over-threshold stimulus thus manifests itself not as increased intensity of the action potential but as an increase in action potential frequency (rate).

AP propagation is unidirectional because the opposite side of the membrane is in the refractory period.

Propagation of AP and local currents

AP propagates along the membrane as a wave of negativity by means of local currents

timetime

Saltatory conduction

Examples of action potentials

A – nerve fibre, B – muscle cell of heart ventricle;

C – cell of sinoatrial node;

D – smooth cell muscle.

A – nerve fibre, B – muscle cell of heart ventricle;

C – cell of sinoatrial node;

D – smooth cell muscle.

SynapseSynapse

Definition

• Synapse is a specific connection between two neurons or between neurons an other target cells (e.g. muscle cells), which makes possible transfer of action potentials.

We distinguish:• Electrical synapses (gap junctions) – close

connections of two cells by means of ion channels. They enable a fast two-way transfer of action potentials.

• Chemical synapses – more frequent, specific structures, they enable one-way transfer of action potentials.

Mitochondrion

Vesicles

Synaptic gap (cleft)

Synaptické mediátory (neurotransmitery)

• The most frequent mediators (neurotransmitters) of excitation synapses are acetylcholine (in neuromuscular end plates and CNS) and glutamic acid (in CNS). Both compounds act as gating ligands mainly for sodium channels. Influx of sodium ions inside the cell evokes a membrane potential change in positive sense – towards a depolarisation of the membrane (excitation postsynaptic potential).

• Gamma-amino butyric acid (GABA) is a neurotransmitter of inhibitory synapses in brain. It acts as a gating ligand of chloride channels. Chloride ions enter the cell and evoke so a membrane potential change in negative sense – membrane hyperpolarization results (inhibitory postsynaptic potential)

Summary• Electric phenomena on biological membranes play a key role in

functioning of excitatory tissues (nerves, muscles)• Resting membrane potential (correctly: membrane voltage) is a

result of a non-equal distribution of ions on both sides of the membrane.

• It is maintained by two basic mechanisms: selective permeable ion channels and by transport systems – both these systems have protein character

• Changes of membrane voltage after excitation are denoted as action potentials

• Membrane undergoes two phases after excitation: depolarization – connected with influx of sodium iions into the cell - and subsequent repolarization – connected with efflux of potassium ions from the cell

• In the refractory period, the membrane is either fully or partly insensitive to stimulation

• Synapse is a connection of two cells which enables transmission of action potentials

„Only two things are infinite, the universe and human stupidity, and I am not sure about the former“.

Albert Einstein (1879-1955)

Authors: Vojtěch Mornstein, Ivo Hrazdira

Language collaboration: Carmel J. Caruana

Presentation design: -

Last revision: September 2009